1. Field of the Invention
The present invention relates to a method and apparatus for feedback control of an air-fuel ratio in an internal combustion engine having two air-fuel ratio sensors upstream and downstream of a catalyst converter disposed within an exhaust gas passage.
2. Description of the Related Art
Generally, in feedback control of the air-fuel ratio in a single air-fuel ratio sensor (O.sub.2 sensor) system, a base fuel amount TAUP is calculated in accordance with the detected intake air amount and detected engine speed, and the base fuel amount TAUP is corrected by an air-fuel ratio correction coefficient FAF which is calculated in accordance with the output signal of an air-fuel ratio sensor (for example, an O.sub.2 sensor) for detecting the concentration of a specific component such as the oxygen component in the exhaust gas. Thus, an actual fuel amount is controlled in accordance with the corrected fuel amount. The above-mentioned process is repeated so that the air-fuel ratio of the engine is brought close to a stoichiometric air-fuel ratio. According to this feedback control, the center of the controlled air-fuel ratio can be within a very samll range of air-fuel ratios around the stoichometric ratio required for three-way reducing and oxidizing catalysts (catalyst converter) which can remove three pollutants CO, HC, and NO.sub.x simultaneously from the exhaust gas.
In the above-mentioned O.sub.2 sensor system where the O.sub.2 sensor is disposed at a location near the concentration portion of an exhaust manifold, i.e., upstream of the catalyst converter, the accuracy of the controlled air-fuel ratio is affected by individual differences in the characteristics of the parts of the engine, such as the O.sub.2 sensor, the fuel injection valves, the exhaust gas recirculation (EGR) valve, the valve lifters, individual changes due to the aging of these parts, environmental changes, and the like. That is, if the characteristics of the O.sub.2 sensor fluctuate, or if the uniformity of the exhaust gas fluctuates, the accuracy of the air-fuel ratio correction amount FAF is also fluctuated, thereby causing fluftuations in the controlled air-fuel ratio.
To compensate for the fluctuation of the controlled air-fuel ratio, double O.sub.2 sensor systems have been suggested (see: U.S. Pat. Nos. 3,939,654, 4,027,477, 4,130,095, and 4,235,204). In such a double O.sub.2 sensor system, another O.sub.2 sensor is provided downstream of the catalyst converter, and thus another air-fuel ratio operation is carried out by correcting delay time parameters of an air-fuel ratio operation of the upstream-side O.sub.2 sensor with the output of the downstream-side O.sub.2 sensor. That is, in a single O.sub.2 sensor system, the switching of the output of the upstream-side O.sub.2 sensor from the rich side to the lean side or vice versa is delayed for a definite time period thereby stabilizing the feedback control, but such a definite time period is variable in the above-mentioned double O.sub.2 sensor system. In this double O.sub.2 sensor system, although the downstream-side O.sub.2 sensor has lower response speed characteristics when compared with the upstream-side O.sub.2 sensor, the downstream-side O.sub.2 sensor has an advantage in that the output fluctuation characteristics are small when compared with those of the upstream-side O.sub.2 sensor, for the following reasons:
(1) On the downstream side of the catalyst converter, the temperature of the exhaust gas is low, so that the downstream-side O.sub.2 sensor is not affected by a high temperature exhaust gas.
(2) On the downstream side of the catalyst converter, although various kinds of pollutants are trapped in the catalyst converter, these pollutants have little affect on the downstream-side O.sub.2 sensor.
(3) On the downstream side of the catalyst converter, the exhaust gas is mixed so that the concentration of oxygen in the echaust gas is approximately in the equilibrium state.
Therefore, according to the double O.sub.2 sensor system, the fluctuation of the output of the upstreamside O.sub.2 sensor is compensated for by a feedback control using the output of the downstream-side O.sub.2 sensor. That is, even when the upstream-side O.sub.2 sensor is deteriorated, the emissions such as HC, CO, and NO.sub.x can be minimized by the correction of the delay time parameters by the output of the downstream-side O.sub.2 sensor. Actually, as illustrated in FIG. 1, in the worst case, the deterioration of the output characteristics of the O.sub.2 sensor in a single O.sub.2 sensor system directly effects a deterioration in the emission characteristics. On the other hand, in a double O.sub.2 sensor system, even when the output characteristics of the upstream-side O.sub.2 sensor are deteriorated, the emission characteristics are not deteriorated. That is, in a double O.sub.2 sensor system, even if only the output characteristics of the downstream-side O.sub.2 are stable, good emission characteristics are still obtained.
In the above-mentioned double O.sub.2 sensor system, however, even when the engine is in a special state, such as a lean air-fuel ratio requesting state, a transient state, a deceleration state, or an idling state, the air-fuel feedback control by the downstream-side O.sub.2 sensor is not suspended, thereby deteriorating the fuel consumption, the drivability, and the exhaust emission characteristics.
For example, in a reductive atmosphere, components such as hydrogen sulfide (H.sub.2 S) are generated by the catalyst to generate an unpleasant odor. This odor is not a problem at a relatively high speed, but, at a relatively low speed, it becomes a problem for passengers in cars following behind. For this purpose, in the prior art, at a relatively low speed such as in an idling state or a low load state, the air-fuel ratio is forcibly made lean, thereby reducing this unpleasant odor (see: Japanese Unexamined Patent Publication (Kokai) No. 59-103941). However, even when such a forcible lean air-fuel control is carried out by a air-fuel ratio feedback control by the upstream side O.sub.2 sensor, a fuel increment is carried out by the air-fuel ratio feedback control by the downstream-side O.sub.2 sensor, so that it is impossible to obtain a lean air-fuel ratio, thus generating the above-mentioned unpleasant odor. Also, when the controlled air-fuel ratio by the feedback of the upstream-side O.sub.2 sensor is changed rapidly from a lean air-fuel ratio to a stoichimetric air-fuel ratio, the feedback by the downstream-side O.sub.2 sensor may not follow the change of the air-fuel ratio, thereby making the air-fuel ratio rich, and thus deterioratig the fuel consumption, the drivability, and the exhaust emission characteristics.
Also, in a transient state, such as a rapid acceleration/deceleration state, a gear-switchover state, or a take-off state, the controlled air-fuel ratio is greatly changed due to asynchronous fuel injection, the delay of the response speed of the feedback control by the upstream-side O.sub.2 sensor, and the like. Therefore, in a transient state, when the feedback control by the downstream-side O.sub.2 sensor is carried out, the air-fuel ratio is overcorrected. As a result, immedietely after the engine leaves such a transient state, the controlled air-fuel ratio is overrich or overlean, thereby deteriorating the fuel consumption the drivability, and the exhaust emission characteristics.
Further, when an idling state of the engine continues for a long time, the activity of the downstream-side O.sub.2 sensor is reduced compared with that the upstream-side O.sub.2 sensor, due to the difference therebetween in location, heat mass, and the like. When the activity of the downstream-side O.sub.2 sensor is lost, a flow-out type O.sub.2 sensor output processing circuit (see: FIG.3A) generates a lean sir-fuel ratio output, and a flow-in type O.sub.2 sensor output processing circuit (see: FIG. 3B) generates a rich air-fuel ratio output. Therefore, when the base air-fuel ratio is lean and an idling state continues, the air-fuel ratio is first brought to the rich side by the feedback control of the downstream-side O.sub.2 sensor. Then, in the case of a flow-out type circuit, correction is performed upon the rich air-fuel ratio, thus making it overrich, while in the case of a flow-in type circuit, correction is performed upon the rich air-fuel ratio, thus moving it towards the lean side, until finally, the air-fuel ratio becomes overlean.
On the other hand, when the base air-fuel ratio is rich and an idling state continues, the air-fuel ratio is first brought to the lean side by the feedback control of the downstream-side O.sub.2 sensor. Then, in the case of a flow-in type circuit, correction is performed upon the lean air-fuel ratio, thus making it overlean, while in the case of a flow-out type circuit, correction is performed upon the lean air-fuel ratio, thus moving it towards the rich side, finally, the air-fuel ratio is overrich. Accordingly, in a long duration idling state and thereafter, the controlled air-fuel ratio is overrich or overlean, thus deteriorating the fuel consumption the drivability, and the exhaust emission characteristics.